Production of acetylene with an arc heater

ABSTRACT

In the processes, an electric arc is produced in a confined area between two annular electrodes and caused by magnetic field generated forces exerted thereon to describe a generally axially extending annular path as it moves substantially continuously around and between the electrodes. Process gas is admitted into the confined area through a substantially circumferential path radially external to the electrodes from which the process gas passes in a generally radial direction through the gap between electrodes and through the annular path described by arc movement, is pyrolized, and moves downstream toward an exhaust nozzle separated from the arc zone by elongated fluid-cooled heat shields and so distant from the arc zone that, according to one process, process gas pyrolized at a predetermined temperature has a mass flow rate such that the gas is cooled to a temperature at which the desired recombination product is present in substantial proportion when it reaches the nozzle. In other processes, quenching gas different from the process gas is added to the pyrolized process gas at one or more axial positions along the path of movement of the pyrolized process gas. In other methods, additional process gas is added to the pyrolized process gas at one or more axially spaced positions along the length of the confined area.

Maniero et al.

[451 Oct. 10, 1972 1 PRODUCTION OF ACETYLENE WITH Primary Examiner-Edward J. Meros AN ARC HEATER Attorney-A. T. Stratton, C. L. McHale and M. l. Hull [72] Inventors: Daniel A. Maniero, Pittsburgh;

Charles B. Wolf, Irwin, both of Pa. [571 ABSTRACT [73] Assigneez Westinghouse Electric Corporation In the processes, an electric arc 1s produced in a conpittsburgh, fined area between two annular electrodes and caused by magnetic field generated forces exerted thereon to Flled! 1969 describe a generally axially extending annular path as 2] A N 870 472 it moves substantially continuously around and 1 pp 0 between the electrodes. Process gas is admitted into Related US. Application Data the confined area through a substantially circumferential path radially external to the electrodes from [62] 2: :3 7 3 2 1966 which the process gas passes in a generally radial direction through the gap between electrodes and through the annular path described by are movement, g1. .bzgc/s zazri is pyrolized and moves downstream toward an [58] Field 260/679 haust nozzle separated from the arc zone by elongated 48/197 fluid-cooled heat shields and so distant from the arc zone that, according to one process, process gas pyrolized at a predetermined temperature has a mass [56] References Cited flow rate such that the gas is cooled to a temperature UNITED ATE P NTS at whtiih thea1 desired recombination prtilducthis preselnt 1n su tanti proportion w en 1t reac es t e nozz e. 3,445,191 5/1969 Brumng et a1. ..23/277 In other processes, quenching gas different from the 3389189 6/1968 Hirayarpa et 2 process gas is added to the pyrolized process gas at 3,168,592 2/1965 Crchelli et a1 ..260/69 one or more axial positions along the path of move- Baddour e o the py o e process In other m Orbach process ga is added o he py oli ed 3514264 5/1970 Sennewald et 0/ process gas at one or more axially spaced positions along the length of the confined area.

3 Claims, 2 Drawing Figures I25 :24 38- I 44 I36 n33 I26 2 I22 92 48 I49 I29 I42 1 if Q 64 I? 'oq I 69 42' lo! we :3 II 12 3 4 [3| I32 l2? e c I55 n9 I 97 99 ea 899l fi l0 50 u Ins ms 114 I we I44 I30 26 a 4 '24 as 32 me 836 |6| 4| 34 29 33 ,56 Q] IOZ/ l m n 29 I08 .7: ll? i|34 I23. '0 2 7 48 4957 i/: 95

, .4- I39 5 .4 5 5 2 SNO? n2 as eal gol tesgg 42 u% ISI '-l7 2| IE2 -|52 '53, SOURCE PATENTEUUCI 10 1372 SHEET 1 0F 2 wumnom 8mm 8 0. m2 mm. m.

cooled electrodes 11 and PRODUCTION OF ACETYLENE WITH AN ARC HEATER CROSS-REFERENCE TO RELATED APPLICATIONS This application is a division of application Ser. No. 527,787, filed Feb. 16, 1966, now US. Pat. No. 3,522,015.

This invention relates to improvements in chemical processing, and more particularly to chemical processing utilizing an improved arc heater for the processing of one gas and the conversion thereof to another desired product gas, or carbon.

It has been known for some years that an electric arc may be used to pyrolize a gas, and after the gas has been decomposed into atoms and free radicals, the atoms and free radicals may recombine to produce a different and desired product gas, depending upon the temperature to which the decomposed gas is quenched or cooled and the speed with which it is cooled after pyrolysis takes place. Sometimes an auxiliary gas may be added to assist in rapid cooling. 1

The are heater of our invention utilizes an arc to pyrolize the process gas or feed stock, for example CH or C H which is thereafter quenched and cooled, so that a substantial yield of the product gas, for example C H is obtained. The are heater of our invention is especially suitable for handling high flow rates of the process gas, and for producing the product gas with a minimum of kilowatt hours of electricity to the arc per pound of the product gas. For example, in one experimental test run of our arc heater, methane (Cl-l was utilized as a process gas with a flow rate of 0.275 lbs. per second. The power to the arc electrodes was 840 kilowatts, and the acetylene, C l-I yield was 5.03 kilowatt hour of electricity utilized by the arc, per pound of acetylene C 11 a figure which compares very favorably with other processes now in general use, such as the DuPont process and I-luels process.

Accordingly a primary object of our invention is to provide a new and improved direct conversion chemical processing are heater.

Another object is to provide a new and improved chemical processing are heater in which large flow rates of a process gas may be employed.

Still a further object is to provide a new and improved arc heater for chemical processing in which the kilowatt hours per pound of a product gas are maintained at a low value.

These and other objects will become more clearly apparent after a study of the specification, when read in connection with the accompanying drawings, in which:

FIG. 1 is a cross-section through the direct conversion chemical processing arc heater according to the preferred embodiment of our invention; and

FIG. 2 is a graph illustrating the operation of the apparatus of FIG. 1.

Referring now to the drawing of FIG. 1 for a more detailed understanding of the invention, a pair of fluid 12 have an are 13 therebetween. It is seen that the electrodes 11 and 12 are annular in shape, and that each of the electrodes has an annular magnetic field producing coil therein, these being designated 14 and 15 respectively. If desired, field coils 14 and 15 are energized by direct current, the leads to the coil 14 being shown at 16 and 17 passing through a passageway 18, the lead to coil 15 being shown at 21 and 22 passing through passageway 23. Coils l4 and 15 may be energized by direct current with their fields in opposition so that a magnetic field is produced between electrodes which is substantially transverse to the path of the are 13 and which exerts a force on the are 13 which causes it to rotate substantially continuously around the annular arcing surface of the electrodes in a conventional manner. The rotation of an are by a magnetic field has been described elsewhere inthe literature of the art and in prior art patents and need not be described in detail.

Coil 14 is seen to be mounted in an annular housing 24 composed insulating material, the housing 24 being disposed within an annular cup-shaped member 25 which is generally U-shaped in cross section, which has ends 26 and 27 thereof abutting against annular shoulders 28 and 29 of an electrode supporting and fluid channeling member 30. The cup-shaped member 25 is seen to be spaced from the inner wall of the generally U-shaped electrode 11 providing fluid passageways 31, 32 and 33. It is seen that the portion of the fluid passageway 31 at the left-hand end thereof communicates with a fluid header 34, which may be a fluid inlet header, and which has fluid inlet 35. Passageway 33 around the other side of the coil and coil housing communicates by way of a plurality of circumferentially spaced passageways 36 and 37 with a fluid header 38 which may be a fluid outlet header. In actual practice in the construction of the arc heater of the Figure, two fluid inlets spaced l apart are provided for the fluid inlet header 34, there being an inlet in addition to the inlet shown at 35; and in addition there are two fluid outlets 39, only one being shown for simplicity of illustration for the fluid outlet header 38, the fluid outlets being preferably spaced 180 apart.

The internal construction of the other electrode 12 is similar to that of the electrode 11 and need not be described in detail. Fluid passageways 41, 42 and 43 around the three sides of the field coil 15 communicate, one with fluid outlet header 44 and another with fluid inlet header 45. Fluid inlet header 45 is seen having the inlet passageway 46 communicating therewith, and outlet header 44 has outlet 47.

Between the two electrodes 11 and 12 there is a heat shield enclosing the chamber in which the arc takes place, this heat shield including two generally annular ring members 51 and 52, separated by a center ring 53. The two annular L-shaped ring members generally designated 51 and 52 each has a plurality of annular fingers 54 and 55 respectively extending from ring inside wall portions 48 49 respectively forming in each ring member a plurality of spaced annular passageways 62 and 63 respectively. Within 360 annular grooves 19 and 20 of ring members 51 and 52 respectively are disposed ring members 9 and 10 respectively having short fluid inlet headers 56 and 57 respectively, fluid header 56 having inlet 58, and fluid header 57 having inlet 60. Disposed 180 from the aforementioned inlets 58 and 60, are two outlets 59 and 61 communicating with fluid outlet headers and 166 in rings 9 and 10 respectively, so that fluid flows in two semicircular paths in passageways 62 and 63.

Between the aforementioned ring member 51 and the adjacent electrode 11 there is disposed means for introducing gas, either a process gas or a quenching gas or some auxiliary gas, at a plurality of circumferentially spaced points around the electrode 11, the gasentering the chamber 50 through the annularpassageway 64. The annular passageway or opening 66 constitutes a gas header, and in actual practice the arc heater would employ two gas=inletsto this gas header 66, one of these gas inlets being shown at 67, it being understood that another gas inlet spaced if desired at 180 therefrom would also be provided. From the gas header 66 gas passes through {a plurality of circumferentially spaced passageways 68 and into annular space 69, thence through gaps or spaced bores 70 into the aforementioned annular space 64 and into the portion of the chamber 50 where the arc rotates.

Adjacent the aforementioned electrode 12 is a similar-gasheader 71 having gas inlet 72, gas from the gasheader 71 entering the arc chamber through the annular space 73. t

It is noted that electrode 11 is separated from annular ring 51 by annular insulating means including annular insulating members 74 and 75, and that electrode 12 is electrically insulated from ring member 52 by means including annular insulating members 77 and 78.

It is noted that the remainder of the arc heater includes sectionalized arc chamber wall, these including a heatshield generally designated 80, which section may be eliminated if desired, a heat shield generally designated 81, and a heat shield generally designated 82. It is further noted that the right-hand end of the arc chamber 50 i is closed by an end plug generally designated .84 and that on the left-hand of the arc heater as seen in the Figure there is a nozzle generally designated 86..

It is seen that the aforementioned cylindrical heat shield 82 is fluid cooled by fluid flowing in passageways 88 and 89,'passageway 88 connecting with fluid header 91 and thence with fluid inlet 92, passageway 89 communicating with fluid header 94 and thence with fluid outlet 95.

It is further to be noted that gas is admitted into the chamber 50 at a plurality of circumferentially spaced positions around the heat shield 82, there being an annular insulating member 97 with spaced bores'98 communicating with an annular passageway 99 which communicates by way of spaced bores 101 with an annular gasheader 100 which has a gas inlet, not shown for convenience of illustration, disposed at a convenient position on the are heater.

Gas. is also admitted at a plurality of circumferentially spaced positions around the end plug generally designated 84 and near the inner chamber wall of the heat shield 82. There is an annular space 102 which serves as an auxiliary gasheader, and an annular gasket of insulating material 103 having a plurality of bores 104 at spaced intervals therearound to permit a quenching gas or an auxiliary gas or a process gas to be introduced into the chamber 50. The annular space 102 communicates with annular gas header 106. It is further seen that the heat shield generally designated 82 is electrically insulated from the electrode 12 by means 107, 108, and that the head shield generally designated 82 is electrically insulated from the end plug 84 by means111 and 112 composed of insulating material. End lug 84 is seen to be fluid cooled, having a conical passageway 113 extending from fluid header 114 connected to fluid inlet 115. The conical passageway 113 communicates with a fluid header 116 connected to fluid outlet 1 17.

The aforementioned heat shield 81 is similar to the heat shield 82 and need not be described in detail. Suffice it to say regarding the heat shield 81 that the surface thereof which faces the arc chamber 50 is fluid cooled by fluid passageways having fluid inlet and fluid outlet headers communicating with fluid inlets and fluid outlets. A gas is admitted at a plurality of circumferentially spaced positions between heat shield 81 and heat shield and a gas is admitted at, or can be admitted at, a plurality of circumferentially spaced positions between heat shield 81 and electrode 1 1, the last named gas passing through the passageway 119 and through spaced bores 120 in an annular ring of insulating material 121. The gas header for the last named gas is designated 122 having inlet 124, while the gas header for gas admitted by way of annular space 155 the heat shields 80 and 81 is designated 123, having a gas inlet, not shown for convenience of illustration.

Particular attention is directed now to the heat shield 80, which is electrically insulated from heat shield 81 by insulating means 125 and 126 and electrically insulated from nozzle 86 by insulating means 128 and 129. Heat shield 80 is cooled by fluid passing through annular passages 131 communicating by way of holes 127 with a fluid header 132 having fluid inlet 133, and communicating by holes with outlet header 144 having fluid outlet 134, spaced l80 from inlet 133 so that fluid flows through two semicircular paths. Passageways 136 and 137 may be used for seeding or sampling or quenching or mixing purposes, but may be plugged up by plugs 138 and 139, which may be secured thereto by bolts, not shown for convenience of illustration.

If desired, the heat shield 80 may be entirely eliminated from the arc heater, and the arc chamber wall may comprise an up-stream heat shield 82, and .a down-stream heat shield 81.

Gas may be injected between the; aforementioned 1 heat shield 80 and the aforementioned nozzle 86 by way of gas header 141 having inlet 142, header 141 communicating by spacedpassageways i fi with annular space 162.

The aforementioned nozzle 86 is' see to include a fluid cooled inner surface 146 having fluid low passageway 147 near the inner surface, the passageway 147 communicating with a fluid inlet header 148 and a fluid outlet header 149, these communicating with inlets and outlets respectively, not shown for convenience of illustration.

ltis seen, then, that there is provided an arc heater with means for supplying a large electrical current :to the electrodes, symbolized by leads 151 and 152 connected to source of potential. 153 to produce and sustain the arc 13; magnetic field coils14 and 15 are energized to set up a magnetic fieldwhich causes the arc to rotate at a predetermined speed which is neither too fast nor too slow, the are moving from any particular position before the intensely hot are spot has burned through the electrode, the are not returning to the same position until that area or that point on the electrode has had a chance to cool down to a safe temperature. It

is further that gas may be admitted into the arc chamber at a plurality of points depending upon the process gas used and the desired product. As will be i of rings 51-52 of various widths available for use.

Particular reference is made now to Tables I, II, III and N which show the results of tests run. In all of these test runs, the arc was powered by alternating current, and the electrode gap was inch.

To insure purity of process gas and accurate measurements of the recombination products, the arc chamber was flushed before process gas was admitted by supplying chemically pure nitrogen to the chamber for several seconds. At the conclusion of the supplying of process gas to the arc heater, nitrogen was again supplied to the arc heater for several seconds.

The tables show results for various flow rates of the process gas and various powers to the electric arc.

Tables 1 and 11 show a chemical analysis of the gas in the arc heater where methane is used as the process gas, and the desired recombination product is acetylene C H In obtaining test samples of gas for chemical analysis, a probe consisting of a water cooled concentric copper tube with a Va inch bore was employed, the bore opening of the probe being at the nozzle, preferably of the axis thereof. Runs 1, 2 and 3 are the same runs in Tables I and II with their result analyzed in different manners and show the results for different flow rates of the methane, and different input powers to the arc heater, together with the quantity of the desired output (Table I), measured in kilowatt hours per pound of the desired recombination product gas. Test run number 3 shows that for a certain flow rate and are input power, the yield of acetylene, measured in kilowatt hours per pound of acetylene, compares very favorably with presently used processes. In test run NO. 3 which produced such efficiency in the conversion of Methane to acetylene, Methane was introduced at the following points in the arc heater:

40 percent by gas inlet 67.

40 percent by gas inlet 72.

4 percent by gas inlet 124 and gas header 122.

4 percent by the corresponding gas inlet (not shown) for gas header 100. 4 percent by the gas inlet (not shown) for gas header 4 percent by gas header 123 and gas inlet 110.

4 percent by gas inlet 142 and gas header 141.

Particular reference is made now to Table 11, where test runs 1,2 and 3 are analyzed with respect to, or with reference to, the total carbon, and it is seen that the electrical and flow rate conditions which produce the most efficient conversion to acetylene are also accompanied by the production of the least carbon in the arc heater.

Particular reference is made now to Graph No. 1, where variations in some of the products are chartered for various flow rates.

In the curves of FIG. 2, the quantities of H CH,, C, and CgHg expressed in Mole percentage are shown varying with variations in the reciprocal of the flow rate of CH, supplied to the arc chamber, and the curves correspond generally to the values of Tables 1 and I1.

Particular reference is made now to Table No. 111, where the process gas employed was C H and the composition of the product is analyzed with respect to the three different flow rates and three different input powers to the arc heater.

Particular reference is made nowto Table No. 1V, where product values obtained during the same test runs, that is runs 4, 5 and 6, are shown with percent reference to the production of total carbon, where the process gas is as before C H In summary, we have provided an arc heater in which a process gas may be introduced at one or more of a number of points up-stream and down-stream of an arc to assist in providing maximum production of the desired product for a given kilowatt hours of power to the arc.

Further, although an auxiliary quenching gas was not employed in the test runs described, our apparatus provides that a quenching gas or an auxiliary gas may be introduced at a number of points up-stream and downstream of an arc or substantially in the area of the arc path.

These auxiliary or quenching fluids, including gases and liquids, may be desirable or needed in other chemical conversion processes for which our apparatus is adapted. 1

We have provided a direct conversion chemical processing arc heater which, in terms of kilowatt hours per pound of the product gas desired yields far greater efficiency than any known method of gas conversion.

This invention constitutes a further development and advance in the chemical processing art, as described in the copending application of Messrs. Bruning, Kienast, Kemeny and Hirayama for Cross-Flow Arc Heater Apparatus and Process for the Synthesis of Carbon, Acetylene and Other Gases, filed Nov. 12, 1965, Ser. No. 507,345 new U.S. Pat. No. 3,554,715; to the application of C. Hirayama et al for Method and Equipment for the Pyrolysis and Synthesis of Hydrocarbons and Other Gases and Arc Heater Apparatus for Use Therein," Ser. No. 446,012, filed Apr. 6, 1965, now issued U.S. Pat. No. 3,389,189; that of P. F. Kienast et al for Arc Heater Apparatus for Chemical Processing, Ser. No. 471,914, filed July 14, 1965, now U.S. Pat. No. 3,445,191; that of A. M. Bruning et al for Arc Heater Apparatus for Chemical Processing, Ser. No. 471,914, filed July 14, 1965 now U.S. Pat. No. 3,345,191; and that of D. A. Maniero et al for Process for Hydrogen Cyanide and Acetylene Production in An Arc Heater Having A Rotating Arc, Ser. No. 657,867, filed Aug. 2, 1967-, now U.S. Pat. No. 3,460,902, all of the above-identified applications and patents being assigned to the assignee of the instant invention.

Whereas we have shown and described our invention with respect to apparatus which may be employed which gives satisfactory results, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense; changes may be made and equivalents substituted without departing from the spirit and scope of the invention.

I rate second in an are power of from about 360 to about 840 TABLE I.-COMPOSI'IION IN VOLUME PERCENT OF PRODUCT GAS IN CH4 FLAME CH4, Power Composition, percent Kw. Run lbs./ to gas, hI'./lb. No. See. kw. Hz CO CH4 02H: CzH C4112 other 02112 1 0.045 360 54.93 33.39 10. m 1.05 0.23 0.24 9.9 2 0. 081 440 60.98 38.17 9.52 ass 0.21 0. 25 6.8 3 0.275 840 35.80 1.47 53. 59 7.82 0.83 0.29 0.20 5.03

TABLE II-CONVERSION 0F CH4 To PRODUCTS cent passes through the arc path adjacent the first elec- Percent reference to total carbon trode, substantially 4 percent of the total methane Run N0 Soot Go Other input being introduced in a substantially eircular path' 5 36 2 34 2 between the second electrode and the ad acent end of 5.'24 249 III: 3'01 the second elongated heat shield, an additional sub- 7.70 5.30 6.02

99?! ave s the Qt?! met input being TABLE III.COMPOSITION IN VOLUME PERCENT OF PRODUCI GAS IN C Ho FLAME CaHt, Power Composition, percent;

lbs./ to gas, Run N0 Sec. kw. Hz CO CH4 CzHz Cal-I4 CgHs C H2 CoHu other 4 a- 0. 106 630 )6. 71 1. 52 0. 1.16 O. 15 0. 12 0. O8 5 0.035 330 34 16. 69 3. 22 3. 0. 31 0.13 6 0.061 39. 36 6. 97 12. 07 3. 80 36. 11 0.29 0, 16 0, 67

TABLE .IV.--CONVERSION 0F C3Hb TO PRODUCTS Percent reference to total carbon We claim as our invention:

'1. Improved chemical process for using arc heater apparatus comprising means forming an arc-chamber, said arc chamber being defined by eight axially disposed elements including in order a closure plug, a first elongated heat shield, an upstream electrode,.a heat shield, a second electrode axially spaced from the first electrode, the last named heat shield enclosing the space between the first and second electrodes, a second elongated heat shield extending from the second electrode in a downstream direction, a third elongated heat shield extending from the second elongated heat shield in a downstream direction, and an exhaust vent for the arc chamber at the end of the third elongated heat shield remote from the electrodes, with means for introducing a process fluid at seven positions between the eight axially spaced elements, both the first and second electrodesbeing generally annular in shape with means for producing andsustaining an are between the first and second electrodes and means for causing the arc to move substantially continuously in an annular path around and between the first and second electrodes, introducing methane into the arc chamber to be converted by pyrolization into atoms and free radicals and then by recombination into acetylene at a mass flow from about 0.045 to about 0.275 pounds per kilowatts, substantially 40 percent of the methane being. introduced at a plurality of peripherally spaced positions between the heat shield and the second electrode whereby said 40 percent of the methane passes through the arc'path near the arcing surface of the second electrode, introducing an additional substantially 40 percent of the methane at "a plurality of peripherally spacedpositions between the heat shield and the first electrode whereby said additional 40 perintroduced between the second elongated heat shield and the third elongated heat shield, and a still further substantially 4 percent of the total methane input being introduced in a substantially circular path between the nozzle means and the adjacent end of the third elongated heat shield for cooling the atoms and free radicals to the temperature conducive to their recombination into acetylene.

2. A chemical conversion process using an arc heater having electrodes producing an arc rotating in an annular path with extended fluid cooled arc'chamber walls between the electrodes and the nozzle, comprising the steps of introducing methane gas through the arc path into the arc chamber, and adjusting the mass flow rate of methane to a rate of about 0.275 pounds per second in an arc power of from about 840 kilowatts, pyrolizing the methane into a gas mixture comprising atoms and free radicals, moving the gas mixture by the mass flow through the arc chamber at such. a rate that the gas loses an amount of heat to the cooled chamber walls, and thereby cooling the gas mixture to a temperature-at which the atoms and free radicals recombine into acetylene when it reaches the nozzle.

3. A chemical conversion process comprising the steps of utilizing an electric arc to heat methane gas in an enclosure having an exhaust nozzle, and moving the methane at a flow rate of about 0.275 pounds per second in an are power of about 840 kilowatts, pyrolizing the methane into a gas mixture comprising atoms and free radicals, moving the gas mixture away from the arc zone, and cooling the gas by conduction and convection to the enclosure at such a rate that upon reaching the nozzle it is cooled to a temperature at which the atoms and free radicals recombine into acetylene. 

2. A chemical conversion process using an arc heater having electrodes producing an arc rotating in an annular path with extended fluid cooled arc chamber walls between the electrodes and the nozzle, comprising the steps of introducing methane gas through the arc path into the arc chamber, and adjusting the mass flow rate of methane to a rate of about 0.275 pounds per second in an arc power of from about 840 kilowatts, pyrolizing the methane into a gas mixture comprising atoms and free radicals, moving the gas mixture by the mass flow through the arc chamber at such a rate that the gas loses an amount of heat to the cooled chamber walls, and thereby cooling the gas mixture to a temperature at which the atoms and free radicals recombine into acetylene when it reaches the nozzle.
 3. A chemical conversion process comprising the steps of utilizing an electric arc to heat methane gas in an enclosure having an exhaust nozzle, and moving the methane at a flow rate of about 0.275 pounds per second in an arc power of about 840 kilowatts, pyrolizing the methane into a gas mixture comprising atoms and free radicals, moving the gas mixture away from the arc zone, and cooling the gas by conduction and convection to the enclosure at such a rate that upon reaching the nozzle it is cooled to a temperature at which the atoms and free radicals recombine into acetylene. 